1. Field of the Invention
The invention relates to rotary compressors, or roots-type blowers, of the backflow type, and more particularly, to reducing noise associated with roots blowers employed as superchargers for internal combustion engines.
2. Background of the Invention
A roots blower is known in the prior art, for example EP 0 174 171 B1. Rotary compressors of this type are used, for example, in motor vehicles to convey compressed air to the internal combustion engine. The air is enclosed in a transfer volume on the inlet side of the pump between vanes of the axial rotors and the pump casing and is conveyed to the outlet side of the pump. During the conveyance, no volume change of the transfer volume takes place, and thus, no pressure rise. The outlet sides is at a higher pressure, so that when the transfer volume is opened toward the outlet side, a backflow of the volume of fluid in the transfer volume occurs, and hence the gases are pressurized.
In the prior art, the nature and quantity of the backflow in roots blowers is influenced by backflow orifices. These orifices are designed to reduce noise and vibration that are known to occur in such blowers due to the unsteady nature of the pressurization process. To achieve the desired result, various positions, lengths, and widths of the usually slot-shaped backflow orifices are proposed in the prior art. One such example is shown in EP 0 174 171 B1 in which an approximately V-shaped backflow orifice is disclosed. In the prior art, the opening of the backflow orifices are located parallel to the blower""s casing.
The inventors of the present invention have recognized that the noise reduction measures, as presented in the prior art, do not satisfactorily control supercharger noise. Measurements of the dynamic pressure at the outlet of roots blowers coupled to internal combustion engines indicate resonances at various compressor speeds. These resonances arise at the basic orders of the roots blower, e.g., 3rd, 6th, 9th, 12th, and harmonic orders, excite a specific characteristic mode within the blower due to acoustic coupling of the transfer volume of the compressor to the under pressure inlet system of the engine via the backflow orifices.
Drawbacks of the prior art are overcome by a rotary compressor, preferably a roots blower, having an inlet, an outlet, two axially parallel rotors, engaging one into the other, for conveying a transfer volume, enclosed between vanes of the rotors and the pump casing, from the inlet to the outlet, including a backflow orifice arrangement attached to the pump casing in the region of the outlet for exchanging fluid with the transfer volume. An edge of the backflow orifice arrangement is at a varying height from an inner surface of the pump casing. Preferably, the height of the backflow orifice varies continuously, an example of which is a linear variation in height. In one embodiment, the height of the edge is at a minimum closer to the outlet and at a maximum at a farther from the outlet.
In contrast to known roots blowers, the present invention discloses that the outlet of the backflow orifice does not lie in a surface parallel to the blower""s casing. Instead, it projects, at least partially, beyond such the blower""s casing. Such a variation in the height of the edge of the backflow orifice has a damping influence on the noise generated by the compressor. The exact geometry of the outlet can be optimized by testing.
According to the invention, the roots blower has at least one backflow orifice, the edge of which is at a varying distance or height from the inner casing contour. The variation in the height of the edge occurs continuously, that is, without jumps or discontinuities. For example, the height of the edge from the casing may increase linearly or in a ramp-like manner from a minimum distance to a maximum distance.
An advantage of the present invention is that a roots blower, according to the present invention, has lower operating noise. This diminution of noise occurs at resonant orders of the blower. Furthermore, the shape of the edges of the backflow orifice has been shown to reduce the sound level over the 8500 to 12000 rpm speed range.
Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification.